Apparatus for automatically adjusting electrical networks



Sept. 25, 1945. LEHDE 2,385,454

APPARATUS FOR AUTOMATICALLY ADJUSTING ELECTRICAL NETWORKS Filed July 17,1941 PatentedSept. 25, 1945 APPARATUS FOR AUTOMATICALLY ADJUST- INGELECTRICAL NETWORKS Henry Lehde, Brooklyn, N. Y., asslgnor to The BrownInstrument Company, Philadelphia, Pa 1 a corporation of PennsylvaniaApplication July 17, 1041, Serial No. 402,00:

4. Claims. (c1. ire-23s).

This invention relates to apparatus for automatically balancing oradjusting electrical networks. The invention can be used to balance anydirect current circuit or network where the direction or sign of a smallvoltage unbalance is an indication oi the direction oi the adjustment tobe made. In this capacity it is particularly useful in potentiometerarrangements employing thermocouples to indicate, record or controltemperatures, or {or the measurement or control oi any eiiect that canbe associated with or produce.

direct current voltages.-

It is the primary objector this invention to provide a very inexpensive.simple and rugged apparatus for automatically operating or balancing adirect current potentiometer, or other network, where the presence of asmall voltage unbalance is utilized to control the apparatus. Whilevarious terms automatic potentiometers are known, it has heretoiore beennecessary to employ a galvanometer or similar expensive and delicateinstrument to control the motor or other means which automaticallybalances the potentiometer. The present invention retains thesensitivity and accuracy oi galvanometer control, but is comparativelymuch more raised in construction and stable in operation.

Another object is to provide such apparatus which is not ailected bypositioning or by moderategnounts of shock.

Another obiect is to provide such apparatus which is or simple andcompact construction thereby to permit the apparatus to be made inportable form.

A iurther important object is to provide a sim-' ple and inexpensivemeans for supplying the un varyinc direct current required in the slidewire of a potentiometer. This has previously been a serious drawback tothe use of potentiometer arrangements, requiring both the maintenance 0!a battery for supplying the current, and a standard cell or other meansior periodically checking its value. v

The invention will be described with reference to a potentiometercircuit for measuring direct current voltages, and also with referenceto the conventional Wheatstone bridge for resistance measurements. Itsadaptation to the adjustment oi! similar networks will be obvious.

In the accomp ny l drawing:

Fig.1 is a diagrammatic representation oi a circuit embodying theinvention as applied to the usual potentiometer arrangement formeasuring the voltage generated by a thermocouple.

Fig. 2 is a similar view illustrating the embodiment of the invention ina circuit including a Wheatstone bridge.

In the form of the invention shown in Fig. 1, a thermocouple 8 is shownas arranged in a potentiometer circuit. The numeral 9 represents acontact arm last to a shaft II and traversing a slide wire resistancell, one terminal it or which is connected with the thermocouple I. Thethermocouple I is also connected to a brush it engaging a slip ring itflxed to the shalt it or a small alternator indicated generally at it.

Current is supplied to the slide wire ii by a small direct currentgenerator II. This generator is driven by a synchronous motor issupplied with constant regulated current from power'lines 2| and II andthis generator has a permanent magnet field, provided by permanentmagnets 22, instead of an electromagnetic ileld.

A ballast resistor 23 and an adiustable resistor 24 are connected inseries with the slide wire resistance II and with the commutator oi thedirect current generator it.

The alternator ll includes a rotating held it one terminal or which isconnected with the slip ring I4 and the other terminal oi which isconnected with another slip ring it fixed to the shalt ii. This secondslip ring II is engaged by a brush 32 which is connected with thecontact arm I traversing the slide wire resistance ii. The alternator IIalso includes stationary armature coils 33 and 34 which are connected inserice and are connected with an amplifier I. The amplified current fromthe amplifier 35 is led to one winding ll oi a two phase motor indicatedgenerally at 4|. The other winding 4! or this motor is permanentlyenergized through connection with the main power lines 20 and ii. Themotor 4| is preferably constructed generally as illustrated, including aconducting disk II last to the motor shaft 40, the conducting-disk beingpreierablymade of aluminum to obtain; a low inertia eliect. Thisconducting disk is mad in the adjoining magnetic fields oi the twoseparately energized alternating current coils and 42. A motor soconstructed is very simple and inexpensive and also the motor will notrun when one phase is deenergized as is the case with more eflicient twophase motors. Thus, when the coil 4| is deenergized, the coil 4! acts asan electromagnetic brake to stop the disk ll and prevent overshooting.

The shalt it of the two phase motor ll is connected by a pair of gearsll and 48 with the shaft ll carrying the contact arm I. This shalt IIalso carries a pointer II which indicates the voltage onan adjacentscale II.

In operation, the voltage drop between the contact arm I and theterminal ll of the slide wire resistance ll opposes the voltagegenerated by the thermocouple 8. The current in the slide wireresistance ii is furnished by the small direct current generator I!driven by the synchronous motor ll.

Since the direct current generator I! has a permanent magnet field andis driven at a constant speed when the synchronous motor is is suppliedwith constant regulated current from lines 20 and 2|, th direct currentgenerator IE will deliver constant voltage to the ballast resistor 23,the adjustable resistor 24 and the slide wire resistance The currentdelivered by the direct current generator I 8 will also remain constantas long as the resistance in its circuit remains constant. The purposeof the ballast resistor 23 is to minimize the efiect of fluctuation inthe resistance of the generator I! due to temperature changes. Thustemperature changes cause slight variation in the resistance oi thegenerator l8, but as the value of the ballast resistor 23 isconsiderably higher than the resistance of the generator l8 and iscomposed of a material which is not materially aflected by temperaturechanges, a slight variation of the resistance of the generator l8, dueto temperature changes, does not cause an appreciable change in thetotal circuit resistance. The current in the slide wire resistanc ii istherefore maintained substantially constant, the exact value desiredbeing obtained by a permanent adjustment of the adjustable resistor 24.No further calibration or adjustment is necessary to maintain theaccuracy of the instrument.

Whenever an unbalance occurs in the voltage drop between the contact arm8 and the terminal l2 the slide wire resistance H and the thermocouple8, current will fiow through the rotating field 30 oi. the alternator 6.Since the alternator I6 is driven by the synchronous motor la, themagnitude and phase 0! the altemating voltage generated in thestationary field coils 33 and 34 will depend upon the magnitude anddirection of the current unbalance flowing through the rotating field 30of the alternator IS. The output of the alternator I6 is amplified toany desired degree by the amplifier 35 and is then led to the winding 40of the two phase motor 4|. The other winding 42 of this motor ispermanently energized by the same alternating current supply whichdrives the synchronous motor IS. The phase adjustment is such that thevoltage delivered by the amplifier 35 to the m0- tor 4| is 90 or 270'out of phase with the voltage supply, depending upon the direction ofthe current flowing through the alternator field 30.

By means of the gears 41, 48, the motor 4| can rotate the shaft it onwhich is mounted the contact arm 9 and the pointer 50. The phase or thecurrent supplied by the amplifier 35 to the motor 4| determines thedirection in which this motor turns. The connections are made so thatthe motor 4| will run in a direction to adjust the voltage drop acrossthe slide wire resistance H to equal that of the thermocouple. Thepointer 50 indicates the voltage on the adjacent scale The circuit mayreadily be adapted to any of the usual forms of apparatus for recordingtemperatures, or any other variables dependent on or associated withvoltage. The recording element, which maybe a pen or stylus, or someform of printing mechanism, can be controlled by the motor 4|, while amoving chart driven by a constant speed motor or by clockwork, can bearranged in the usual way to furnish the time element.

In the event it is desirable for control or regulating purposes to driveauxiliary equipment from the shaft I0, it may be necessary to replacethe motor 4| by another motor having more power. By means known in theart, the amplifier 35 can be connected to thyratron tube circuits tocon- 'trol a reversible motor of any desired size. Or a phase shiftrelay can be connected in place of motor 4| to control a able manner.

It is important that no iron be used in the field structure of thealternator i6, or hysteresis efiects willmask the true effect of theunbalance current in the field coil 30. The stationary armature coils 33and 34 should also be shielded from the effects of any stray alternatingfields.

A simplified form of the invention may be used for balancing aWheatstone bridge, or similar direct current network, as shown in Fig.2. The conventional Wheatstone bridge comprises the ratio resistances R1and R2, the known resistance RA and the unknown'Rx, connected to thebattery B as shown. For convenience R1 and R2 are selected in valuesthat are multiples of 10. The mid points of the bridge, instead of beingconnected to s. galvanometer, are connected to the brushes and 6|engaging slip rings 62 and 83, respectively, these slip rings being fastto the shaft 64 of an alternator indicated generally at 85. Thisalternator 65 is similar to the alternator l6, having a rotating field66 one terminal of which is connected with the slip ring-62 and theother terminal of which is connected with the other slip ring 63, andthe alternator 65 also having stationary armature coils 61 and 88 whichare connected in series and are connected with an amplifier 69. Thealternator S5 is driven by a synchronous motor 15 supplied with constantregulated current from power lines 16 and 11.

The amplified current from the amplifier 44 is led to one winding 8|)01' a two phase motor indicated generally at 18. The other winding 18 ofthis motor is permanently energized through connection with the mainpower lines 18, 11. The motor 18 is preferably constructed similar tothe motor 4|, having a conducting disk 8| fast to the motor shaft 82,the conducting disk being preferably made of aluminum to obtain a lowinertia effect. This conducting disk Ii is arranged in the adjoiningmagnetic fields oi the two separately energized alternating currentcoils l8 and 80.

The shaft 82 of the two phase motor 19 is connected by a pair of gears83, 84 with a shaft 44 carrying a contact arm 86 traversing theresistance Rs. This shaft is also shown as carrying a pointer 81 whichindicates the resistance on an adjacent scale 88.

To balance the bridge, the resistance RA must be adjusted so that novoltage appears between the mid points of the bridge, when Ii anunbalance voltage does appear at the mid points of the bridge, itsdirection depends on whether the resistance RA is too large or toosmall. This voltage is impressed upon the rotating field 66 of thealternator 65, driven by the synchronous motor 15, and causes thealternator to generate a voltage whose phase depends on whether theresistance RA is too large or too small.- This voltage is amplified bythe amplifier 89 and is led to the winding of the two phase motor 19.The other winding 18 is connected to the same alternating current supplylines 16 and 11 which supply current to the synch'ronous motor 15. Aswith the form of the invention shown in Fig. 1, the connections are madeso that the motor 18 will run in a direction dependent upon themagnitude oi the resistance reversible motor in any 'suit- Rn. By meansof the gears 83 and 84, the motor 19 can rotate the shaft 85 on which ismounted the contact arm 86 and the pointer 81. The contact arm 85 isthen moved on the variable resistance Ra until this resistance Ra isadjusted to the proper value, which is indicated by the pointer 81 onthe scale 88.

It will be evident that a number of changes can e made in both forms ofthe invention without departing from the principles described. Thedirect current voltage caused by unbalance of the network can be used inother ways to produce an alternating current of reversible phase. Theunbalance voltage can be connected to the stationary coils of thealternator, producing a stationary field structure. The rotating coil ofthe alternator then becomes the armature, and the slip rings areconnected to the input of the amplifier. Or, the alternating currentinput to the amplifier can be gen- 'erated by simply interrupting ormodulating the unbalanced direct current voltage by mechanical orelectronic means at a frequency corresponding to that of the supplyvoltage.

In place of driving the alternator by a synchronous motor, it may bedriven by any source of mechanical power, whether of constant speed ornot, provided a second small alternator is driven in synchronism withit. The output of the second alternator is then used to energize thecoil of the two phase motor which is supplied with line current, thisbeing the coil 42 in the form of the invention shown in Fig. 1 and thecoil 18 in the form of the invention shown in Fig. 2.

The generator i8 and its control resistances 23- and 24 can, of course,be used as a current supply in any potentiometer circuit, regardless ofthe method of balancing the circuit. However, for very precise work, asin laboratory measurements, the generator l8 may be replaced by the.conventional current supply battery and standard cell circuits. Theautomatic balancing arrangement, comprising alternator l6, amplifier I5,and motor 41 can be retained to facilitate rapid and accuratemeasurements.

From the foregoing it will be seen that the principles of the inventioncan be applied in a number of ways to produce the desired results,

and the application of the invention is therefore not to be limited tothe specific embodiments shown and described but is to be accordedcurrent with the phase thereof dependent upon the direction of theunbalance between said voltages, means for amplifying said alternatingcurrent, and means actuated by said amplified alternating current forreducing said voltage unbalance.

2. In an electrical network for balancing an unknown direct currentvoltage, means providing a source of unvarying direct current andcomprising a direct current generator having a permanent magnet field,means for driving said generator at a constant speed, a relatively highresistance connected in series with said generator and regulating saiddirect current, a second resistance in series with said high resistanceand said generator, said second resistance having an adjustable tappedportion, means for connecting said unknown direct current voltage inseries opposition with the voltage drop across said adjustable tappedportion of said second resistance, an alternator having afield, meansfor energizing said field in response to any voltage unbalance, asynchronous motor driving said alternator, means for amplifying theoutput of said alternator, and means actuated jointly by said amplifiedalternating current and the the voltage supply to said synchronous motorfor reducing said voltage unbalance.

3. In an electrical network for balancing an unknown direct currentvoltage, means providing a source of unvarying direct current andcomprising a direct current generator, means for driving said generatorat a constant speed, a resistance in series with said generator, saidresistance having an adjustable tapped portion, means for connectingsaid unknown direct current voltage in series opposition with thevoltage drop across said adiustable tapped portion of said resistance,means for generating an alternating current with the phase thereofdependthe full range of equivalents comprehended by the accompanyingclaims.

I claim as my invention:

1. In an electrical network for balancing an. unknown directcurrentvoltage, means providing a source of unvarying direct current andcomprising a direct current generator having a permanent magnet field,means for driving said generator at a constant speed, a relatively highresistance connected in series with said generator and regulating saiddirect current, a second resistance in series with said high resistanceand said generator, said second resistance having an adjustable tappedportion, means for connecting said unknown direct current voltage inseries opposition with the voltage drop across said adjustable tappedportion of said second resistance, means for generating an alternatingcut upon the direction of the unbalance between said voltages, means foramplifying said alternating current, and means actuated by saidamplified alternating current for adjusting the adjustable tappedportion of said resistance to reduce said voltage unbalance.

4. In an electrical network for balancing an unknown direct currentvoltage, means providing a source of unvarying direct current andcomprising a direct current generator, means for driving said generatorat a constant speed, a resistance in series with said generator, saidresistance having an adjustable tapped portion,

means for connecting said unknown direct current voltage in seriesopposition with the voltage drop across said adjustable tapped portionof said resistance, means for generating an alternating current with thephase thereof dependent upon the direction of the unbalance between saidvoltages and comprising a stationary winding and a movable winding,means for energizing one of said windings in response to voltageunbalance to produce an alternating current in the other winding withthe phase thereof dependent upon the direction of the unbalance between.said voltages, means for amplifying the output alternating current fromthe alternator, and means actuated by said amplified altemating currentfor reducing the voltage unbalance.

HENRY LEHDE.

